Raise RuntimeError instead of using the nonexistent self.error

Fixes #136.
Merge djmitche/build-relengapi:issue136 (PR #136)
Return a non-HTML description for errors
Introduce a 'relengapi' module and 'restapi' service

This service handles display of error messages automatically, avoiding
the situation where a button just does nothing, with the error hidden in
the developer console.